1. Field of the Invention
The present invention is related to a flash memory, and more particularly, to a method for accessing flash memory and associated controller and memory device.
2. Description of the Prior Art
Flash memory can be electrically erased and programmed for data storage. It is widely applied in memory cards, solid-state drives, portable multimedia players, etc. As the flash memory is a non-volatile memory, no power is needed to maintain the information stored in the flash memory. In addition, the flash memory provides fast read access and better shock resistance. These characteristics explain the popularity of the flash memory.
The flash memories may be categorized into NOR-type flash memories and NAND-type flash memories. Regarding the NAND flash memory, it has reduced erasing and programming time and requires less chip area per cell, thus allowing greater storage density and lower cost per bit than the NOR flash memory. In general, the flash memory stores data in an array of memory cells made from floating-gate transistors. Each memory cell can store one bit of information or more than one bit of information by adequately controlling the number of electrical charge on its floating gate to configure the threshold voltage required for turning on the memory cell made of a floating-gate transistor. In this way, when one or more predetermined control gate voltages are applied to a control gate of the floating-gate transistor, the conductive status of the floating-gate transistor would indicate the binary digit(s) stored by the floating-gate transistor.
Flash memory can be categorized into Single-Level Cell (SLC), Multiple-Level Cell (MLC) or Triple-Level Cell (TLC) according to the type. In a TLC flash memory, each memory cell can be used to store three bits, so each memory cell supports eight write voltage levels. However, if there are quality problems when manufacturing flash memory and makes a portion of memory cells can't supports eight write voltage levels entirely (for example some higher voltage levels can't be written), or a portion of voltage levels of the eight write voltage levels have shift or sticky problems, a data writing error may be occurred, causing the problems of reading data.
In addition, if the manufacturer discards these flash memories which have quality problems, it also causes the waste of costs.